Synthesis of Diiron(I) Dithiolato Carbonyl Complexes - Chemical

Review pubs.acs.org/CR

Synthesis of Diiron(I) Dithiolato Carbonyl Complexes Yulong Li†,‡ and Thomas B. Rauchfuss*,‡ †

School of Chemistry and Pharmaceutical Engineering, Sichuan University of Science & Engineering, Zigong 643000, China School of Chemical Sciences, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States

‡

S Supporting Information *

ABSTRACT: Virtually all organosulfur compounds react with Fe(0) carbonyls to give the title complexes. These reactions are reviewed in light of major advances over the past few decades, spurred by interest in Fe2(μ-SR)2(CO)x centers at the active sites of the [FeFe]-hydrogenase enzymes. The most useful synthetic route to Fe2(μ-SR)2(CO)6 involves the reaction of thiols with Fe2(CO)9 and Fe3(CO)12. Such reactions can proceed via mono-, di-, and triiron intermediates. The reactivity of Fe(0) carbonyls toward thiols is highly chemoselective, and the resulting dithiolato complexes are fairly rugged. Thus, many complexes tolerate further synthetic elaboration directed at the organic substituents. A second major route involves alkylation of Fe2(μ-S2)(CO)6, Fe2(μ-SH)2(CO)6, and Li2Fe2(μ-S)2(CO)6. This approach is especially useful for azadithiolates Fe2[(μ-SCH2)2NR](CO)6. Elaborate complexes arise via addition of the FeSH group to electrophilic alkenes, alkynes, and carbonyls. Although the first example of Fe2(μ-SR)2(CO)6 was prepared from ferrous reagents, ferrous compounds are infrequently used, although the Fe(II)(SR)2 + Fe(0) condensation reaction is promising. Almost invariably low-yielding, the reaction of Fe3(CO)12, S8, and a variety of unsaturated substrates results in C−H activation, affording otherwise inaccessible derivatives. Thiones and related CScontaining reagents are highly reactive toward Fe(0), often giving complexes derived from substituted methanedithiolates and C−H activation.

CONTENTS 1. Introduction 1.1. Scope of Review 1.2. Physical Properties of Diiron(I) Dithiolato Carbonyls 1.3. Chemical Properties of Diiron(I) Dithiolato Carbonyls 1.4. Structures of Diiron(I) Dithiolato Carbonyls 1.4.1. Bis(monothiolate) Derivatives 1.4.2. Chelating Dithiolate Derivatives 2. Synthesis of Diiron(I) Dithiolato Carbonyls from Iron(0) Reagents 2.1. Reactions of Iron(0) Reagents and Thiols 2.1.1. Monothiols 2.1.2. Mechanism of Conversion of Iron Carbonyls and Thiols to Fe2(μ-SR)2(CO)6 2.1.3. Derivatives of Alkanedithiols and Alkenedithiols 2.1.4. Derivatives of Aryldi- and Arylpolythiols, Including Heterocycles 2.1.5. Derivatives of Elaborated Alkanedithiols (Mainly Propanedithiolates) 2.1.6. Routes to Fe2(μ-SR)2(CO)6 via [Fe2(μSR)(CO)7]− 2.2. Reactions of Iron(0) Reagents and Organic Di- and Polysulfides 2.2.1. Ordinary Disulfides 2.2.2. Functionalized Disulfides © 2016 American Chemical Society

2.2.3. Peri-naphthalene Disulfides 2.2.4. Perthioesters 2.2.5. From Polysulfides 2.3. Reactions of Iron(0) Reagents and Thioethers 2.3.1. Unstrained Thioethers 2.3.2. Mechanistic Aspects of Thioether Activation by Iron(0) Carbonyls 2.3.3. Strained Thioethers: Thiiranes and Thietes 2.4. Reactions of Iron(0) Reagents and CSContaining Reagents 2.4.1. Thiones 2.4.2. Derivatives of CS2 2.4.3. Dithioesters and Related Di- and Trithiocarbonates 2.4.4. Thioamides 2.4.5. Miscellaneous Thiocarbonyl Derivatives 2.5. Reactions of Iron(0) Reagents and Thioesters and Thioamides 2.6. Reactions of Iron(0) Reagents, Elemental Sulfur, and Alkenes 2.6.1. Alkene, Alkyne, and Imine Trapping Reactions 2.6.2. Diene-Trapping Reactions

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Received: November 13, 2015 Published: June 3, 2016 7043

DOI: 10.1021/acs.chemrev.5b00669 Chem. Rev. 2016, 116, 7043−7077

Chemical Reviews 2.7. Transmetalation of Heterometal Dithiolates by Iron(0) Carbonyls 2.8. Reactions of Iron(0) Reagents with Miscellaneous N−S Compounds 3. Synthesis of Diiron(I) Dithiolato Carbonyls from Iron(I) Precursors 3.1. Alkylation of Li2Fe2(μ-S)2(CO)6 3.1.1. Reaction of Li2Fe2(μ-S)2(CO)6 with Organic Halides 3.1.2. Reaction of Li2Fe2(μ-S)2(CO)6 with Organometallic Electrophiles 3.2. Alkylation of Fe2(μ-S2)(CO)6 3.3. Addition of Alkenes and Alkynes to Fe2(μSH)2(CO)6 3.4. Reactions of Fe2(μ-SH)2(CO)6 with Aldehydes and Ketones 3.5. Reactions of Fe2(μ-SH)2(CO)6 with Cyclic Imines 3.6. Photoaddition of Unsaturated Compounds to Fe2(μ-S2)(CO)6 3.7. Conversions of Other Diiron(I) Compounds 4. Synthesis of Diiron(I) Dithiolato Carbonyls from Iron(II) Precursors 4.1. Abiological Pathways 4.2. Biological Pathways 5. Conclusions Associated Content Supporting Information Author Information Corresponding Author Notes Biographies Acknowledgments Abbreviations References

Review

of the diiron dithiolate active site has been partially elucidated in the past few years.4,5 The chemistry of synthetic high-spin Fe−S clusters has been reviewed extensively, e.g., by Holm and co-workers.6,7 In contrast, low-spin species of the type Fe2(μ-SR)2(CO)6−xLx have not been reviewed, although they have been the subject of hundreds of publications.8 Because the literature is so scattered and in view of the intensity of work on these compounds, a review is timely. These complexes are chemically versatile for many reasons. First, these complexes are so easy to make that they are almost unavoidable in organoiron chemistry. A huge number of thiolates can be installed (see Supporting Information). The hexacarbonyl complexes readily undergo stepwise substitution by many ligands, thus multiplying the range of possible complexes well into the thousands. The resulting electron-rich derivatives exhibit acid−base and redox properties, forming hydrides and many oxidized derivatives both by chemical and electrochemical means.9 Reviews on the preparation of models for the [FeFe]hydrogenases have been published,10−12 but the basic organometallic chemistry behind these models has never been emphasized. The chemistry of iron carbonyls with chalcogen ligands has an extensive history,13−15 as do the reactions of organosulfur compounds with iron and other metal carbonyls.16

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1.2. Physical Properties of Diiron(I) Dithiolato Carbonyls

The diiron dithiolato hexacarbonyls are bright red−brown diamagnetic solids (Figure 2).17 They are stable in air and often

1. INTRODUCTION 1.1. Scope of Review

This Review summarizes the syntheses of compounds of the type Fe2(μ-SR)2(CO)6. Such compounds have come under intense scrutiny because of their structural similarity to the active site of [FeFe]-hydrogenases, which are some of the fastest catalysts for oxidation and production of H2 (Figure 1, right).1 Crystallographic analyses of the [FeFe]-hydrogenase enzymes were reported in 1998−1999,2,3 and the biosynthesis

Figure 2. Sample (20 g) of crystalline Fe2(μ-pdt)(CO)6.

sublimable, and some even survive steam distillation.18 Thermogravimetric analysis19 shows that, in the presence of atmospheric oxygen, Fe2(μ-SEt)2(CO)6 decomposes around 100 °C with loss of CO and formation of FeS and Fe.18 In an early report, Fe2(μ-SEt)2(CO)6 and Fe2(μ-SPh)2(CO)6 were purified by boiling for 1 h in dilute aqueous NaOH.20 Further illustrating the robustness of these compounds, Fe2(μSMe)2(CO)6 is prepared at 120−30 °C and 65 atm of CO (see Section 2.2.1). Most derivatives are soluble in organic solvents, although water solubility can be achieved21 through the use of hydrophilic substituents, e.g., the tetraol Fe2[μSCH2CH(OH)CH2OH]2(CO)622 and the sodium sulfonate Fe2[μ-S2C2H3(CH2SO3Na)](CO)6.23 Because of their convenient stability and solubility, Fe2(μ-SR)2(CO)6 complexes are usually purified by chromatography, as illustrated by separation of two isomers of Fe2(μ-SMe)2(CO)6.24 Although these compounds are probably toxic, they have been considered in the pharmaceutical context.21

Figure 1. Structure of Fe2(μ-SR)2(CO)6 and the H-cluster. 7044


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1.3. Chemical Properties of Diiron(I) Dithiolato Carbonyls

organometallic chemists, including Hieber, were very confused about the structures of the metal carbonyls. The crystallographic analysis of Fe2(μ-SEt)2(CO)6 in 196274 was the first of hundreds of subsequent structural studies. These studies reveal that diiron(I) dithiolato hexacarbonyls adopt very similar structures. The Fe2S2 butterfly core has idealized C2v symmetry, with the Fe−Fe bond and two apical CO ligands defining one plane of symmetry. With the bulky thiolate SC6H2-2,4,6-(i-Pr)3, the Fe−Fe distance contracts to 2.466 Å,75 substantially shorter than that observed in the SEt species (2.537 Å)74 and SPh species (2.516 Å).76 1.4.1. Bis(monothiolate) Derivatives. Depending on the orientation of the R substituents, the compounds Fe2(μSR)2(CO)6 can adopt three structures: equatorial−equatorial (ee), axial−equatorial (ae), and axial−axial (aa), as depicted in Figure 4.77 The aa isomer is only observed with chelating

A vast literature describes the reactions of the diiron dithiolato hexacarbonyls. The diiron dithiolato hexacarbonyls are electrochemically silent over a wide potential range. The [Fe2(μSMe)2(CO)6]+/0 couple is ∼0.8 V vs [Fe(C5H5)2]+/0, and the reduction [Fe2(μ-SPh)2(CO)6]0/− is −1.4 V vs the same reference.9 The oxidation potential shifts by >1 V up to replacement of CO with donor ligands,25 and the reduction can be adjusted by as much as 0.5 V through judicious selection of the dithiolates.26 The hexacarbonyls are stable to all but the strongest nonoxidizing acids.27 The most commonly studied reactions of the diiron dithiolato hexacarbonyls involve substitution of CO by Lewis bases. Substituting ligands include phosphines, isocyanides, carbenes, cyanide, phosphites, thioethers, and others.11 For highly nucleophilic ligands, disubstitution occurs often near room temperature. Installing less basic ligands requires more forcing thermal or photochemical activation, as well as the use of decarbonylation agents such as Me3NO.28 The substituted derivatives are often rather basic and oxidizable,29 whereas the hexacarbonyls are typically air-stable. The Fe2(μ-SR)2(CO)6 center is relatively immune to reactions directed at the organic substituents on the thiolates. In this way, the Fe2(μ-SR)2(CO)6 center serves as a platform for the development of elaborate, multifunctional structures. Although Fe(0) reagents are highly reactive toward thiols, once formed, the Fe2(μ-SR)2(CO)6 complexes are essentially unreactive toward thiols and thiolates (see section 3.7). Reactions tolerated by the Fe2S2 core in these hexacarbonyls are listed in Table 1. The functional group tolerance also applies to solvents used in syntheses. As is discussed in greater detail below, the most common iron carbonyl reagents are Fe2(CO)9 and Fe3(CO)12, typically employing tetrahydrofuran (THF) and toluene as solvent, respectively. However, for highly polar organosulfur reagents, methanol or aqueous methanol has been employed.23,30 Although Fe2(μ-SR)2(CO)6 centers are fairly chemical resilient, they are attacked by strong electrophiles and strong nucleophiles. The sulfur centers are oxidized with peracid and dioxiranes.63,69,70 S-Alkylation by the strongly electrophilic EtOTf occurs with Fe2(μ-SR)2(CO)4(PMe3)2, but probably less readily with the parent hexacarbonyl.71 The hexacarbonyls decompose upon treatment with aqueous base, although this reaction has not been described. Some alkylthiolate derivatives undergo deprotonation with strong non-nucleophilic bases (see section 3.7). Strong reducing agents such as Co(C5Me5)2 reduce the hexacarbonyls.72

Figure 4. Three isomers of diiron dithiolato carbonyls with CO ligands omitted. The ee, ae, and aa isomers are sometimes referred to as syn eq, anti, and syn ax, respectively.

dithiolates and as a component of Fe2(μ-SH)2(CO)6.78 Typical nonchelating organic thiolates give a mixture of the ae and ee isomers. In the case of Fe2(μ-SMe)2(CO)6, the interconversion of these isomers is so slow that the major (92%) ae isomer can be separated chromatographically from the minor (8%) ee isomer.24,79 The ee isomer is observed in the solid Fe2[μSC6H2-2,4,6-(i-Pr)3]2(CO)6 (Figure 5), but the solution

Figure 5. Structure of Fe2[μ-SC6H2-2,4,6-(i-Pr)3]2(CO)6. With its very bulky thiolates, the Fe−Fe distance is 0.06 Å shorter than that typically observed. Reproduced from ref 75. Copyright 1999 Elsevier.

1.4. Structures of Diiron(I) Dithiolato Carbonyls

Although this Review focuses on synthetic methods, it is useful to review general structural trends. The structure proposed by Reihlen et al. in 1928 is presented in Figure 3.73 Early

structure was not described.75 The ae and ee isomers of compounds Fe2(μ-SMe)2(CO)6 and Fe2(μ-SBn)2(CO)6 undergo carbonyl substitution reactions by phosphine ligands at different rates.77 The isomerization of ee-Fe2(μ-SMe)2(CO)6 to ae-Fe2(μ-SMe)2(CO)6 is intramolecular.80,81 The barrier for isomerization is higher for small alkyl groups, such as Me, Et, and Bn. For t-Bu, the barrier is sufficiently low (17 kcal/mol) that the dynamic interconversion of the ae and ee isomers can be observed by NMR at 100 °C.79 For aryl groups, the ae isomer predominates, and isomerization of ae to ee isomer is 10 times faster than that for small alkyl groups.79

Figure 3. Structure of Fe2(μ-SEt)2(CO)6 proposed by Reihlen et al. in 1928.73 Notice the careful attention to depicting each carbon with four bonds. 7045


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1.4.2. Chelating Dithiolate Derivatives. In the methane-, 1,2-ethane-, 1,3-propane-, and 1,4-butanedithiolates82−84 Fe2[μS2(CH2)n](CO)6, the organic substituents are constrained to the aa conformation. An additional degree of freedom applies to the compounds Fe2[(μ-SCH2)2X](CO)6 (X = CH2, NH, O, etc.), where the dithiolate backbone is nonplanar,17,85−87 being analogous to cyclohexane. In these complexes, the conformational equilibration is fast at room temperature on the NMR time scale, but individual conformers are sometimes observed by low-temperature NMR measurements. For 2-substituted propanedithiolates, Fe2[(μ-SCH2)2CHR](CO)6, the flippamers are not of equal energy, and the R substituent tends to adopt the equatorial orientation. In such cases, “equatorial” refers to the orientation of the R substituent relative to the conformation of the ring, in analogy to the nomenclature used for substituted cyclohexanes. The azadithiolate derivatives Fe2[(μ-SCH2)2NR](CO)6 have additional degrees of freedom (Figure 6).87 When the R

carbonyls. This route is also the best understood. As summarized in the Supporting Information, a large number of compounds have been prepared from monothiols. The first example of this reaction was reported in 1929 by Reihlen et al., who obtained Fe2(μ-SEt)2(CO)6 in good yield by photolysis of a boiling solution of Fe(CO)5 in EtSH.100 These authors demonstrated the evolution of the 1 equiv of H2 and 2 equiv of CO per iron precursor (eq 1). 2Fe(CO)5 + 2EtSH → Fe2(μ‐SEt)2 (CO)6 + H 2 + 4CO (1)

In a series of papers beginning in the 1930s, Hieber, Reihlen’s colleague, refined these findings, beginning a multidecade program focused on the reactions of sulfur compounds with iron carbonyls, especially Fe3(CO)12 (which Hieber referred to as “Fe(CO)4”).101 Early reactions were conducted in neat thiol, although subsequently Hieber conducted related reactions, e.g., using naphthalene-1-thiol, using conventional hydrocarbon solvents.102 He often quantified the evolved gases, with some results being represented in eq 2. 2Fe3(CO)12 + 6RSH → 3Fe2(μ‐SR)2 (CO)6 + 3H 2 + 6CO (R = Et,Ph,1‐C10H 7,2‐C10H 7)

(2)

In the 1970s, Nametkin and colleagues began a multiyear study on organosulfur derivatives of iron carbonyls.18 They examined the reaction of octylthiol with Fe3(CO)12 in heptane solution at room temperature. Under these conditions the diiron complex is produced in 70% yield. The reaction also produces a substantial amount of dioctyl disulfide, the mechanistic significance of which is unexplained. Because disulfides also react with iron carbonyls to give the same dithiolates, their involvement in typical syntheses would not be noticed in many preparations. The RSH/Fe3(CO)12 reaction is currently the most popular route for diiron dithiolato carbonyls.20,61,99,103−116 The RSH/Fe2(CO)9 route to Fe2(μSR)2(CO)6 is also widely employed; these reactions proceed typically at slightly lower efficiency than the Fe3(CO)12 route.18 Because iron(0) carbonyls are specifically reactive toward the SH group, a variety of functionalities can be tolerated on the thiol side chain. Thus, many diiron dithiolato carbonyls have been prepared with reactive functional groups attached to the thiol group. Illustrative of the functional group tolerance is the attachment of an O/N-protected derivative of cysteine (see Table 1). When two such multifunctional thiolates are adjacent on the diiron framework, they undergo an intramolecular condensation to afford an ethanedithiol-derived amino acid (Scheme 1).117 A further example is the efficient preparation of Fe2(μ-SC6H4NH2)2(CO)6 from 4-H2NC6H4SH118 (as well as by hydrogenation of Fe2(μ-SC6H4NO2)2(CO)638). Iron(0) carbonyls are proposed to survive boiling water in their reaction with thiol-containing peptides to give diiron hexacarbonyl dithiolates.119 2.1.2. Mechanism of Conversion of Iron Carbonyls and Thiols to Fe2(μ-SR)2(CO)6. The formation of Fe2(μSR)2(CO)6 from thiols and Fe(CO)5 (and probably Fe2(CO)9, which is a source of Fe(CO)4) proceeds via complexes of the type Fe(CO)4(RSH) and HFe(CO)4(SR) (R = H, Me, Et, Ph). Thiol complexes of other metals have been well-characterized, for example, [Ru(NH3)5(EtSH)]2+.120 The intermediacy of Fe(CO)4(RSH) may also arise by the reaction of H2Fe(CO)4 with thiols, which affords Fe2(μ-SR)2(CO)6 quantitatively at 0

Figure 6. Conformational equilibria for the azadithiolates Fe2(μadtR)L6.

substituent is small (R = H,33 Me,88 CH2CH2OH,89 etc.), the substituent tends to be axial (i.e., projects nearly parallel with the Fe−Fe bond). This geometry is predicted by the anomeric effect, i.e., favorable overlap of the two C−S σ* orbitals and the lone pair on nitrogen. 88 For larger R groups (R = furanylmethyl,90 cycloalkyl,91 etc.), the substituent tends to adopt equatorial orientations. When the substituent is aryl41,92,93 or is unsaturated (e.g., alkynyl42), the amine is planar. In these cases pi-conjugation with the p-orbital of the nitrogen is reflected by short N−C distances. Interesting structural trends emerge for diiron dithiolates obtained from long-chain alkane dithiols. This theme has been systematically examined with dithiols derived from polyethylene glycols (Table 2).84 Shorter chains favor bis(diiron dithiolate)s, whereas dithiols with 14 spacer atoms give 20− 30% yields of diiron dithiolates. These derivatives exist as ae isomers (Figure 7).94−96

2. SYNTHESIS OF DIIRON(I) DITHIOLATO CARBONYLS FROM IRON(0) REAGENTS 2.1. Reactions of Iron(0) Reagents and Thiols

2.1.1. Monothiols. The reactions of iron(0) carbonyls with thiols are the most common route to diiron dithiolato 7046


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Table 1. Reactions of Thiolate Substituents in Fe2(μ-SR)2(CO)6 Complexes transformation

reagent

description

R2NC(O)R′ → R2NC(S)R′ RN(H)C(O)OBu-t → RNH2 R2NSiR3 → R2NH RNH2 → RNHC(O)R′ RNH2 → RNHC(O)R′ ArNO2 → ArNH2 ArI/Br → ArC2−Ar RC2H → RC2(H)N3R′ n RCHCH2 → (RCH−CH2)n n RCCH → (RCCH)n RCHO → RCH2OH ArCHO → porphyrin ROH → ROSiMe3 ROH → RO2CR′ ROH → ROPX2 ROH → RCO2R′ R2CHOH → R2CO quinol → quinone ROH → RBr AcOR → NaOR R2O → RBr RSH → RS−Au R2S → R2SO, R2SO2 RSH → RS(Z)(H)C2H2Z RCl → RS(CO)R′ RCO2H → RCO2M RCO2H → RCO2H−ZnS porph → Zn(porph) Zn(porph) → py-Zn(porph)

(MeOC6H4PS2)2 (Lawesson’s reagent) CF3CO2H CF3CO2H R′COCl N-hydroxysuccinimide, RNH2 H2/Pd/C ArC2H PdCl2(PPh3)2, CuI, Et3N, R′N3, CuI azobis(isobutyronitrile) (AIBN) [Rh(norbornadiene)Cl]2 NaBH4 pyrrole, HOAc, PhCHO, chloranil (C6O2Cl4), BF3·Et2O Me3SiCl R′COCl/Et3N ClPX2 (X = Ph, Cl) R′CO2H, DCC, DMAP Dess−Martin reagent DDQ CBr4/PPh3 NaOMe BBr3 Au dimethyldioxirane maleimide AcSM (M = Na, K) Zr6O4(OH)4(C6H4(CO2)2)6 ZnS Zn(OAc)2 Zn(TPP)

thioamide formation31 t-Boc deprotection32 N-desilylation33 N-acylations34−36 ester amination37 hydrogenation of nitro group38 Sonogashira coupling39−44 azide click reaction45 polymerization of alkenes46 Rh-catalyzed polymerization of alkynes47 aldehydes reduction48 Rothemund condensation, aromatization35,49 O-silylation50 O-acylations34,48,51−53 phosphinite formation53,54 esterification55−58 alcohol oxidation53 quinol oxidation59 Appel halogenation51 ester cleavage60 ether cleavage61 chemisorption62 sulfoxidation63 S-alkylation64 thioester formation35,51 MOF formation65,66 chemisorption67 porphyrin (porph) metalation68 complexation36

Table 2. Yields of Fe2(μ-SR)2 vs [Fe2(μ-SR)2]2 Complexes Derived from Long-Chain Dithiols dithiol

Fe2(μ-SR)2 yield (size of FeS2Xn ring)a

[Fe2(μ-SR)2]2 yield (size of (FeS2Xn)2 ring)a

1,2-C6H4(CH2SH)285 (CH2CH2SH)297 d,l-[CH(OH)]2CH2SH)298 2,2′-[C6Cl2H2SH]299 CH2(CH2CH2SH)295 n and i-PrN(CH2CH2SH)295,96 O(CH2CH2SH)294 HSCH2(CH2OCH2)2CH2SH94 HSCH2(CH2OCH2)3CH2SH94 HSCH2(CH2OCH2)4CH2SH94

85 (7) 0b,c (7) 0b (7) 34 (7) 0b (8) 0b (8) 19 (8) 17 (12) 21 (15) 33 (18)

0b (16) 7 (16) 8 (16) 0b (16) 5 (18) 16−17 (18) 11 (18) 0b (24) 0b (30) 0b (36)

Figure 7. Structure of [Fe2(μ-SC2H4OC2H4S)(CO)6]2. Notice that the organic substituents on sulfur adopt the ae stereochemistry. Reproduced from ref 94. Copyright 2006 American Chemical Society.

a

X = CH2, NR, O. b0% yield is inferred when no product was claimed. 89% of the monomer is obtained by alkylation of Li2Fe2(μ-S)2(CO)6 with 1,4-dibromobutane.83

consistent with the efficiency of the Fe2(CO)9/RSH route to Fe2(μ-SR)2(CO)6 (R = Et, i-Pr, s-Bu).123 Toluene- and 1,3-propanedithiol react with THF solutions of Na2[Fe(CO)4] to produce diiron dithiolato carbonyls in moderate yields.124 This counterintuitive approach probably involves protonation of [Fe(CO)4]2− by the thiol. In contrast to the lability of the HFe(SPh)(CO)4/Fe(CO)4(PhSH) system, the disubstituted complex HFe(SPh)(CO)2[P(OPh)3]2 has been isolated at room temperature and characterized crystallographically.125 This phosphite complex shows, however, no tendency to convert to diiron(I) dithiolates. No such hydride intermediate is observed in the conversion of the Fe(0) phosphine complex Fe(R2PCH2CH2SH)(CO)4, which contains a pendant thiol. No hydride intermediates were observed in the photochemical

c

°C (R = Ph, C6F5, i-Pr).121 The Fe(CO)4(RSH) complexes can be generated by protonation of the thiolato anions [Fe(CO)4(SR)]− (R = Me, Et), which can be obtained by reaction of Fe2(CO)9 and thiolate salts (Scheme 2).122 These alkyl thiol complexes have been characterized by NMR and IR spectroscopy at low temperatures. Upon warming to near room temperature, these Fe(CO)4(RSH) complexes convert to Fe2(μ-SR)2(CO)6 with loss of CO and H2.122 Protonation of PPN[(PhS)Fe(CO)4] gives Fe2(μ-SPh)2(CO)6 even at −78 °C, showing that the acidic thiol complexes convert more rapidly. The facility of the protonation-induced reactions is 7047


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Scheme 1. Formation of Fe2(μ-L-cysOMe−N−C(O)O−tBu)2(CO)6 and Its Intramolecular Condensation117

Scheme 3. Proposed Oxidative Addition Step in the Conversion of Fe(0) Complex with Pendant Thiol to Diiron Dithiolate126

Scheme 4. Role of Diiron(II) Dihydrides in the Formation of Diiron(I) Diphosphides128 conversion to the diiron derivatives Fe2(μSCH2CH2PR2)2(CO)4 (Scheme 3).126 Insight into the pathway connecting Fe(HSR)(CO)4 to the diiron(I) dithiolates is provided by studies on related HX substrates. Sources of Fe(CO)4, generated by photolysis of Fe(CO)5 or dissociation of Fe2(CO)9, react with HX (X = Br, I) to give Fe2(μ-X)2(CO)6.127 The secondary phosphine HP(CF3)2 reacts with Fe2(CO)9 to give the adduct Fe(CO)4[HP(CF3)2], which is analogous to Fe(CO)4(RSH). This monoiron phosphine complex converts thermally and photochemically to the diiron phosphide Fe 2 [μ-P(CF3)2]2(CO)6. Using 1H and 19F NMR spectroscopy, cisand trans-H2Fe2[μ-P(CF3)2]2(CO)6 were characterized as intermediates (Scheme 4).128 Similar intermediates can be envisioned for the formation of dithiolates from RSH. Such dihydrides are poised to eliminate H2 concomitant with formation of the Fe−Fe bond, perhaps after migration of one hydride to a bridging position. Diferrous dithiolato dihydrides are rarely observed.129 The reaction of thiols with the triiron cluster Fe3(CO)12 is an important route to Fe2(μ-SR)2(CO)6 complexes. This conversion differs mechanistically from the pathways starting with Fe(CO)4 sources, although again H2 is produced. In the case of bulky thiols, the reaction of Fe3(CO)12 gives the triiron clusters Fe3(μ-SR)(μ-H)(CO)9 as the first detectable intermediate (R = R = i-Pr, s-Bu, t-Bu, Scheme 5).123,130 The species Fe3(μ-S-tBu)(μ-H)(CO)9 reacts with additional t-BuSH to give the triiron dithiolate Fe3(μ-S-t-Bu)2(CO)9, which retains two Fe−

Scheme 5. Fe3(μ-SR) Intermediates Derived from the Reaction of Fe3(CO)12 with Thiols and Related Reactions123,130,131

Scheme 2. Protonation of [Fe(SR)(CO)4]− Gives Fe(HSR)(CO)4, the Probable Intermediate in the Reaction of Fe(CO)4 Sources with Thiols

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Fe bonds.131 This triiron species appears to be an intermediate connecting Fe 3 (μ-S-t-Bu)(H)(CO) 9 and Fe 2 (μ-S-tBu)2(CO)6.123 Access to Fe3(μ-SR)(μ-SR′)(CO)9 for nonbulky R groups is gained by reaction of Et3NH[Fe3(μSR)(CO)9] (R = t-Bu, C6H11) with sulfenyl chlorides (R′SCl, R′ = Me, Ph). The Fe3(μ-SR)(μ-SR′)(CO)9 clusters convert to the diiron derivatives upon heating (Scheme 5)131 Complementary to the Fe3(CO)12/RSH reactions are reactions of Fe3(CO)12 with thiolate anions (section 2.1.6). The salt Et3NH[HFe3(CO)11], which is produced readily from Et3N, Fe(CO)5, and protic solvent,132 reacts efficiently with thiols according to the stoichiometry in eq 3.18,133,134 Figure 8. Structures of rotated (left) and normal (right) isomers of Fe2[(μ-SCH2)2CEt2](CO)4(dppv). Reproduced from ref 141. Copyright 2013 John Wiley and Sons.

2Et3NH+[HFe3(CO)11]− + 6RSH → 3Fe2(μ‐SR)2 (CO)6 + 2Et3N + 5H 2 + 4CO

(3)

Scheme 6. Conversion of Geminal Enedithiols to Two Kinds of 1,1-Dithiolato Complexes145

2.1.3. Derivatives of Alkanedithiols and Alkenedithiols. Methane-, 1,2-ethane-, and 1,3-propanedithiols react with Fe3(CO)12 to give good yields of Fe2[μ-S2(CH2)n](CO)6.17,28,82 For H2edt and H2pdt, the reactions are accompanied by the formation of ∼5% triiron species of the formula Fe3(μ-xdt)2(CO)7 (xdt = edt, pdt). In the case of Fe2(μ-pdt)(CO)6, yields of 33% and 92% are observed from the reactions of H2pdt with Fe(CO)5 and Fe3(CO)12, respectively.17,28,135,136 King observed that, although H2edt barely reacts with Fe(CO)5,137 it reacts with Fe3(CO)12 to give Fe2(μ-edt)(CO)6 in 65% yield.17 The reactions of substituted iron(0) carbonyls, e.g., Fe(CO)4(PPh3), with thiols have been investigated only lightly, so this approach may be fertile.126,138 In one example, H2pdt reacts efficiently with substituted derivatives of Fe2(CO)9 (eq 4).136

Reactions of Fe 3 (CO)12 with the tetrathiol 1,2,4,5(HSCH2)4C6H2 illustrate the range of reactions for benzylic thiols with Fe(0): formation of dithiolates, dehydrogenation, and desulfurization (eq 5).84 The formation of disulfidecontaining products is consistent with Nametkin’s study of the organosulfur byproducts from the reaction of thiols and Fe3(CO)12.18

Because they are nearly isosteric with the azadithiolate cofactor in the [FeFe]-hydrogenase, propanedithiolate derivatives have been the subject of careful study. Bulky analogues of H2pdt, i.e., R2C(CH2SH)2 (R = Me, Et, i-Pr, and Ph), react with Fe3(CO)12 less efficiently than H2pdt but still produce good yields of the diiron dithiolates.139−141 The diiron complex Fe2[(μ-SCHPh)2SiPh2](CO)6 was obtained by the reaction of meso-Ph2Si(CH(Ph)SH)2 with Fe3(CO)12.142 This and related sterically crowded complexes are of interest as precursors to diiron ditholates that adopt unsymmetrical, “rotated” structures (Figure 8).140,141,143 2-Benzoyldithioacetic acids, as their ene-dithiol tautomers ArC(O)CHC(SH)2, feature geminal dithiol groups and thus represent precursors to diiron dithiolato carbonyls. These dithiols react under mild conditions with Fe2(CO)9 to give the methanedithiolates.144 The same dithiols also react reasonably efficiently with Fe3(CO)12 to produce the Cs-symmetrical alkylidene-1,1-dithiolate complexes. The selectivity for the alkene- vs alkanedithiolate is sensitive to the reactant ratio.145 The differing products in these relatively similar reactions indicate, but do not explain, the involvement of iron hydride intermediates that in turn depend on the Fe(0) precursor (Scheme 6).145

2.1.4. Derivatives of Aryldi- and Arylpolythiols, Including Heterocycles. Aromatic dithiols such as 1,2benzene-, 3,4-toluene, and 1,8-naphthalenedithiols react efficiently with Fe3(CO)12 and related Fe(0) reagents to give the expected diiron derivatives 146 In the reaction of dichlorobenzenedithiol with Fe3 (CO) 12 to give Fe2 (μS2C6Cl2H2)(CO)6,147 an intermediate mixed valence species Fe3(μ-S2C6Cl2H2) 2(CO)7 is produced.148 The structure proposed for this intermediate (Scheme 7) is isomeric with the Fe3(μ-pdt)2(CO)7 species obtained as a minor product in the reaction of pdtH2 and Fe3(CO)12.17 Benzenehexathiol and Fe3(CO)12 react to give a 17% yield of the tris(diiron) derivative of benzenehexathiolate, (μ-C6S6)[Fe2(CO)6]3.149 This species features an inorganic C6S6Fe6 core that should display interesting substitution reactions (Figure 9). An interesting carboxylate-functionalized diiron complex is Fe2[μ-S2C6H2-3,6-(CO2H)2](CO)6, derived from 1,2-dimercaptobenzene-3,6-dicarboxylic acid. The resulting dicarboxylate 7049


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((HSCH2 )2 CHCO2H).55 The carboxylic acid group in Fe2[(μ-SCH2 )2CHCO2H](CO)6 has been converted to amides, including peptides,158 and esters without affecting the diiron core, which remains susceptible to ligand substitution (Scheme 8, see Table 1).55,58,64,159−161 The dihydroasparagusic acid also can be attached to lysine side chains, allowing anchoring of the diiron center to peptides (eq 6).30

Scheme 7. Fe3 Intermediate in the Reaction of Fe3(CO)12 with a Substituted Benzenedithiol148

Scheme 8. Synthesis and Derivatization of Diiron Asparagusic Acid Complex55

Figure 9. Structure of C6S6[Fe2(CO)6]3. Reproduced from ref 149. Copyright 2012 John Wiley and Sons.

forms an organometallic metal−organic framework (MOF) upon condensation with a Zr6 oxide cluster (Figure 10).65,66 The synthesis was achieved in water by displacement of a fraction of benzenedicarboxylate groups in the preformed MOF UiO-66. A hydroxy-functionalized diiron dithiolate is readily prepared from 2-hydroxy-1,3-propanedithiol.17,98 The alcohol group in the resulting complex Fe2[(μ-SCH2)2C(H)OH](CO)6 can be modified in interesting ways, including conversion to a pendant phosphonite ligand (Scheme 9).53 Complementarily, the Scheme 9. Derivatization of Diiron Complexes of 2Hydroxy-1,3-propanedithiolate53 Figure 10. Schematic illustration of the role of the dicarboxylic acid Fe2[μ-S2C6H2-3,6-(CO2H)2](CO)6 as part of a metal−organic framework.65

Heterocyclic dithiols also form diiron dithiolate carbonyls. The diiron hexacarbonyl derivative of dicarboranedithiol 1,2C2B10H10(SH)2 exhibits particularly low reduction potential, reflecting the electron-withdrawing character of the carborane backbone.150 Dithiolato complexes derived from quinoxalinedithiols are of interest because the heterocyclic backbone sustains reversible protonation, thereby affecting the redox properties of the diiron center.151,152 2.1.5. Derivatives of Elaborated Alkanedithiols (Mainly Propanedithiolates). By reactions of functional groups on the thiolate ligands, many derivatives have been prepared employing standard organic synthetic manipulations. This approach has been enabled by the resilience of many organic substituents to substitution reactions at iron, e.g., by phosphines,153−156 cyanide,27 and NHC’s.157 Propanedithiolate complexes bearing functionality at C2 are versatile precursors. One such complex is Fe2[(μ-SCH2)2CHCO2H](CO)6, derived from commercially available dihydroasparagusic acid

hydroxy-1,3-propanedithiol can be modified before the dithiol is installed on the diiron center. In this way, water-soluble diiron dithiolate complexes were produced bearing a sugar residue.60 Propanedithiols functionalized with ligands at the 2-position have been fruitfully pursued en route to biomimetic catalysts.11 Examples of Fe2[(μ-SCH2)2C(Me)CH2R](CO)6 have been prepared where R = SMe,109 NH2, 2-pyridyl, and SAc.110 The hydroxy derivatives Fe 2[(μ-SCH2) 2C(Me)CH2 OH](CO) 6 shows no tendency to decarbonylate, consistent with the weak coordinating tendency of the hydroxyl group.110 The reaction of Fe3(CO)12 with the pyridine-containing dithiol 7050


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[Fe2(μ-SR)(CO)7]−, these species are classified as [Fe(0)]2 derivatives. These anions are prepared by the reaction of Fe3(CO)12 with thiolate anions, often in the form of the mixture RSH/Et3N (eq 8).168−171 The formulas for these monothiolato diiron(0) species are sometimes more explicitly written as [Fe2(μ-SR)(μ-CO)(CO)6]−, which highlights their structural similarity to Fe2(μ-SR)2(CO)6. The salts Et3NH[Fe2(μ-SR)(CO)7] convert to the diiron(I) dithiolates, usually in low yields, upon heating172−174 and with weak oxidants and electrophiles.175−177 These diiron(0) anions are oxidized by S8 to give what are proposed to be diiron(I) anions. Thus, treatment of [Fe2(μ-SEt)(CO)7]− with sulfur followed by methylation with MeI gave both Fe2(μ-SEt)2(CO)6 and Fe2(μSMe)(μ-SEt)(CO)6 (Scheme 12).178 These results point to unexplored pathways for thiolate exchange.175,177

(HSCH2)2C(Me)CH2-2-py results in partial hydrogenation of the ring.162 This unexpected result implicates the intermediacy of iron hydrides that are capable of hydrogenation of the pyridine ring, which normally resists hydrogenation (eq 7). The pendant thioacetate group in Fe2[(μ-SCH2)2C(Me)CH2SAc](CO)6 reacts with 4Fe-4S cubane clusters to give an Fe6S6 ensemble. This cluster coupling reaction proceeds via elimination of AcSEt, followed by displacement of one CO ligand by the pendant 4Fe-4S cluster ligand (Scheme 10).163,164

Scheme 12. Generation and Alkylation of “[Fe2(μSEt)S(CO)6]” as the Et3NH+ Salt

Scheme 10. Stepwise Assembly of a Biomimetic Diiron Dithiolate Complex Containing an Appended 4Fe-4S Cluster (X Is a Trithiolate)163

The diiron(0) complexes [Fe2(μ-SR)(CO)7]− react with sulfenyl chlorides to give the mixed dithiolate in high yields (eq 9).178 The method is, however, limited by the poor availability and instability of most sulfenyl chlorides. Complementary to this route to mixed thiolato diiron complexes is the more reliable reaction of Fe2(μ-S2)(CO)6 with RLi followed by alkylation with alkyl halides (see section 3.1.1).

The reaction of the trithiol (HSCH2)3C(Me)H with Fe3(CO)12 is, not surprisingly, complicated. High Fe(0)/thiol ratios give dendritic derivatives containing three diiron dithiolate subunits.165 When the Fe(0) is the limiting reagent, one obtains the mixed valence species {Fe2[(μ-SCH2)3CMe](CO)4}2 (Scheme 11).166,167 Implicit in these results, but unproven, is the reaction of Fe4 species with additional Fe(0) reagent to give the Fe6 product. 2.1.6. Routes to Fe2(μ-SR)2(CO)6 via [Fe2(μ-SR)(CO)7]−. In addition to the diiron(I) dithiolato carbonyls, diiron(0) monothiolato carbonyls are well-established. With the formula

Et3NH+[Fe2(μ‐S‐t ‐Bu)(CO)7 ]− + EtSCl → Fe2(μ‐S‐t ‐Bu)(μ‐SEt)(CO)6 + CO + Et 3NH+Cl− (9)

The reaction of Fe(0) with di- and trithiols in the presence of base has been developed. Dithiols gives linked pairs of [Fe2(μSR)(CO)7]− centers, which react with bis(sulfenyl chloride)s to give macrocyclic bisdiiron dithiolates (Scheme 13).179,180 The trithiol MeC(CH2SH)3 gives triiron species containing both Fe2(μ-SR)2(CO)6 and [Fe(μ-SR)(CO)7]− centers.181 Such compounds could undergo degenerate intra- or intermolecular

Scheme 11. Tetra- and Hexairon Products from the Reaction of Fe3(CO)12 and MeC(CH2SH)3165−167

Scheme 13. Illustrative Synthesis of Macrocyclic Bis(diiron) Dithiolates (Countercation Not Shown)179,180

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SCF3)2(CO)6”, which, as mentioned elsewhere, could be a perthiolate Fe2(μ-SCF3)(μ-S2CF3)2(CO)6. Sulfido-bridged diferrous carbonyls is an unusual motif,194 although trithiolate related species of the type [Fe2(μ-SR)3(CO)6]+ are wellknown.195 2.2.2. Functionalized Disulfides. The reaction of Fe(0) carbonyls with organic disulfides tolerates many functional groups. For example, treatment of Fe3(CO)12 with bis(2acylaminophenyl)disulfides in toluene at 70 °C affords diiron dithiolates in moderate yields.183 A peptide was conjugated to Fe2(CO)6 by reaction of the cyclic disulfide sandostatin with Fe3(CO)12 (eq 10).196

exchange between the Fe(0) and Fe(I) sites, but such experiments have not been reported. The course of the reactions of MeC(CH2SH)3/Et3N with Fe3(CO)12 contrasts with the related reaction conducted in the absence of Et3N, which afford Fe2(μ-SR)2(CO)6 derivatives exclusively (Scheme 11).165−167 Related to the anionic diiron thiolates, anionic triiron clusters [Fe3(μ-SR)(CO)9]− are also claimed to arise from the reaction of Fe3(CO)12 with NaSR (R = Et, Ph, Bu, etc.) in hot THF.182 Other work implies that some of these proposed Fe3(μ-SR) cluster anions may in fact be [Fe2(μ-SR)(CO)7]−.169 In general, many of these studies implicate the series [Fe(SR)(CO)4]− → [Fe2(μ-SR)(CO)7]− → [Fe3(μ-SR)(CO)9]−. This “aufbau” sequence remains to be tested under well-defined conditions. 2.2. Reactions of Iron(0) Reagents and Organic Di- and Polysulfides

2.2.1. Ordinary Disulfides. Compounds containing RS− SR bonds oxidatively add to iron(0) carbonyls to give diiron dithiolato hexacarbonyls.183−187 The method was first demonstrated by Hieber and Scharfenberg using diphenyldisulfide and Fe3(CO)12.102 The route is typically less efficient than routes from thiols. The photoreaction of Fe(CO)5 with Ph2S2 was originally thought to give an adduct with a Ph2S2 ligand;188 a subsequent crystallographic study confirmed that the product is Fe3(μ-SPh)6(CO)6, i.e., the ferrous species (CO)3Fe(μSPh)3Fe(μ-SPh)3Fe(CO)3.189 The conversion of this green paramagnetic complex to or from Fe2(μ-SPh)2(CO)6 has not been reported. The preparation of Fe2(μ-SMe)2(CO)6 using Me2S2 is instructive. Using the disulfide is more convenient than the thiol route because Me2S2 is a liquid whereas MeSH is a gas. Starting from Fe(CO)5 in neat Me2S2, the yield of Fe2(μSMe)2(CO)6 is 60% when the preparation is conducted under 65 atm of CO (Scheme 14). Without an atmosphere of CO, Scheme 14. Effect of CO Pressure on the Reaction of Fe(CO)5 and Me2S2190

Lipoamide and related derivatives also react efficiently with Fe2(CO)9 to give the corresponding dithiolates, which are chiral (eq 11).197

this reaction affords the red insoluble polymer [Fe(μSMe)2(CO)2]n.190 This and related polymeric ferrous dithiolato carbonyls 191 have not been investigated further. The hexacarbonyl Fe2(μ-SMe)2(CO)6 was found not to react with Me2S2, suggesting that the ferrous product arises via an alternative pathway. The reaction of Fe3(CO)12 with Me2S2 in refluxing benzene gives Fe2(μ-SMe)2(CO)6 in 27% yield.24 The disulfide route has been employed to prepare diiron complexes with relatively exotic thiolates. Oxidative addition of the disilyl disulfide to Fe(CO)5 under UV irradiation gives the silylthiolate Fe2[μ-SSi(t-Bu)3]2(CO)6.192 The volatile Fe2(μSCF3)2(CO)6 was obtained by addition of (SCF3)2 to Fe2(CO)9.193 The related reactions of Fe(CO)5 with XSCF3 (X = SMe, SCF3) give trace amounts of “Fe2(μ-S)(μ-

Because both disulfides and thioethers are reactive toward iron(0) carbonyls, thioether disulfides are expected to give complex product distributions. For example 7-, 8-, and 9membered cyclic thioether disulfides oxidatively add to Fe2(CO)9. The nature of the products depends on the ring size of the precursor. Larger rings appear to favor triiron dithiolates, which are formally related to the diiron dithiolates by the addition of an Fe(CO)4 unit across the Fe−Fe bond (Scheme 15).198,199 Characteristically, these reactions are accompanied by the formation of other products of C−S bond scission such as Fe3(μ-S)2(CO)9, Fe2(μ-S2CH2)(CO)6, and Fe2(μ-edt)(CO)6. The formation of Fe2 vs Fe3 product is subtly based on the flexibility of the thioether backbone.199,200 The Fe3(CO)8 cluster is related to Fe3(μ-S-t-Bu)2(CO)9 described earlier. Tetraalkyl-1,2,4-trithiolanes react with Fe2(CO)9 to give ∼3:1 mixtures of the thiadithiolates160 and the 1,1-dithiolates, with formula Fe2[(μ-SCR2)2S](CO)6 and Fe2(μ-S2CR2)(CO)6, respectively199,200 The thiadithiolates arise via oxidative 7052


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Scheme 15. Reactions of Cyclic Thioether Disulfides with Fe2(CO)9198−200

addition across the S−S bond. The pathway to the 1,1dithiolates is less clear, but they do not result from the thiadithiolates. The reactivity of 1,2,4-trithiolanes is discussed further in section 2.4 in the context of thione substrates. Thiosulfinates, a rare class of organosulfur compounds with the functionality RS(O)SR′, also oxidatively add to Fe(0) carbonyls to give Fe2(μ-SR)2(CO)6 as well as the mono-oxygenated derivative.201 2.2.3. Peri-naphthalene Disulfides. Peri-naphthalene disulfides are particularly popular precursors to diiron(I) dithiolates starting from Fe(0) reagents.146,202,203 The perinaphthalene disulfides, which feature a strained 5-membered C3S2 ring, are readily prepared in one pot by dilithiation of 1bromonaphthalene.202 The resulting Fe2(μ-S2C10H6−xRx)(CO)6 derivatives have simplified stereochemistry relative to the more floppy propanedithiolates. These naphthalene disulfides can be fitted with diverse functional groups, which remain untouched in the oxidative addition of the disulfide by the Fe(0) reagent. Functional groups include Schiff bases,204 porphyrins,29,205 carboxylic acid anhydrides, imides,206 and formyl groups.48 A selection of these products are shown in Figure 11. Naphthalene bis(disulfide) reacts with Fe2(CO)9 in benzene at room temperature to give the bis(diiron) complex [Fe2(CO)6]2(μ-S4C10H4) (eq 12).207

Figure 11. Peri-naphthalenedithiolatodiiron complexes with functional substituents.202,203

Scheme 16. Oxidative Addition of Perthioesters to Fe(0)208,209

carbonyl. The cyclic tetrasulfide derived from tetrafluoroethylene reacts with Fe3(CO)12 carbonyls to afford the volatile Fe2(μ-S2C2F4)(CO)6, a rare example of a diiron perfluoroalkyldithiolate complex. The related reaction, but using the mixed disulfide thioether (C2F4)2S3, is far less efficient, highlighting the greater reactivity of S−S bonds.137 Similar reactions have been mentioned for Me3SiC2H3Sx,216 1,1′ferrocenetrisulfide,217 norbornanetrisulfide,135 and trisulfide of cycloheptadiene218 (Table 3).

2.2.4. Perthioesters. The perthioesters (RC(O)SSR′, RC(S)SSR′) derived from thiosalicylic acids give diiron dithiolates with planar dithiolate bridges.208 The complex Fe2[μ-SC6H4C(O)S](CO)6 is one of the few μ-acylthiolate complexes confirmed by X-ray crystallography. Its reactivity toward nucleophiles would be of interest, because it would appear to be an activated ester. The corresponding reaction of the trithio ester results in a more complicated product resulting from the binding of the thione functionality to a second Fe2(CO)6 subunit (Scheme 16).208,209 This complication arises from the high reactivity of iron carbonyls toward the CS functionality, as is discussed in section 2.4.210−213 2.2.5. From Polysulfides. Organic polysulfides have often been examined as precursors to metal thiolates.214 As applied to the synthesis of diiron dithiolates, the use of organic polysulfides always cogenerates significant amounts of Fe3(μS)2(CO)9.215 This cluster and the diiron dithiolates are readily separated by chromatography so that, in cases where the polysulfide is readily available, there is usually no point in converting it to the dithiol prior to reaction with the iron

2.3. Reactions of Iron(0) Reagents and Thioethers

2.3.1. Unstrained Thioethers. Usually thioethers are less reactive toward diiron carbonyls than disulfides and thiols.219 In 1940, Hieber and Scharfenberg reported, without mentioning yields, that Me2S and Et2S react with Fe3(CO)12 to afford Fe2(μ-SR)2(CO)6 (R = Me, Et).102 Many years later, Nametkin conducted thorough comparison of various organosulfur precursors (Table 4). Only a modest effect on the rate and efficiency of the reaction: The key determinants of yield and rate were the organosulfur and iron(0) reagents. Thiols are the best substrates, and thioethers were the slowest. These results clearly show that the Fe3(CO)12 + RSH route is highest yielding and fastest. Slowest and lowest yielding is the reaction of thioethers with Fe(CO)5.18 The now famous Fe2(μ-pdt)(CO)6 was first prepared by degradation of a tetrathiacyclophane with a refluxing methylcyclohexane solution of Fe(CO)5(Scheme 17).220 The reaction conceivably proceeds via radical pathway, which would be facilitated by the benzylic substituents. Cyclic di- and 7053


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Table 3. Diiron Dithiolates Obtained by Degradation of Organic Polysulfides

Scheme 17. Degradative Routes from Thioethers and Fe(CO)5 to Fe2(μ-pdt)(CO)6220,227

Scheme 18. Routes to the Diiron Methanedithiolate by Fe(0)-Induced Degradation of Methylene Bisthioethers,225,226 Contrasted with the Efficient Thiol Method82

polythioethers appear to more readily degraded to diiron dithiolates, perhaps suggesting that they more readily form adducts, which are poised to degrade via Fe-induced C−S bond scission.221 Rings with several heteroatoms, e.g., the groups −S−CH2−S−C and C−S−CH2−CH2−S−, appear especially susceptible to activation by Fe(0).222−224 The methane- and ethanedithiolate Fe2(μ-S2(CH2)2)(CO)6 arise (no yields being mentioned) from the reaction of Fe3(CO)12 with 1,4-dithiane and 1,3,5-trithiane (SCH2)3, respectively.225 The thiol route (CH2(SH)2 + Fe3(CO)12) is far more efficient (Scheme 18).82,225,226 Another degradative route to Fe2(μ-pdt)(CO)6 involves the combined thermal-photochemical reaction of Fe(CO)5 and 1,3-dithiane.227 Further study is required to reconcile these results, specifically the relationship between the formation of Fe2(μ-pdt)(CO)6 and the product of oxidative addition of the C−S bond. More so than saturated thioethers, usaturated sulfur compounds are activated toward cleavage by Fe(0) carbonyls. The first indication of this reactivity was the finding that vinylthioethers, including thiophenes, oxidatively add to iron carbonyls to give Fe2(μ-SR)(μ-CHCHR)(CO)6 (R = alkyl, vinyl).224,228,229 1,4-Dithiins, six-membered rings of the formula C4R4S2, are activated by Fe2(CO)9 as well as Fe3(CO)12 to give dithiolenes Fe2(μ-S2C2R2)(CO)6, but the yields are low.230 2.3.2. Mechanistic Aspects of Thioether Activation by Iron(0) Carbonyls. C−S bonds typically are strong (∼73 kcal/ mol) and kinetically stable. It is therefore surprising that iron thiolates are often readily obtained in good yields by the reaction of iron carbonyls with unstrained thioethers. The mechanism of this process has not been widely discussed. The simple adducts (R2S)Fe(CO)4 exhibit no unusual structural or

spectroscopic features.203,204 It is conceivable that C−S bond activation requires coordination of the thioether as a bridging ligand. It is reported that organosulfur ligands bridged to several Fe centers are labile toward C−S bond scission.182 Indeed, iron carbonyl clusters with bridging thioether ligands are known. 231,232 For example, the complex Fe 3 (μSC4H8)2(CO)8 is obtained in 30% yield by the reaction of tetrahydrothiophene (THT) with Fe2(CO)9 at room temperature (eq 13).153 The reaction of THT with Fe3(CO)12 at elevated temperatures gives moderate to good yields of Fe3(μS)2(CO)9 and Fe2(μ-S2)(CO)6, together with butane and butenes.233

The photochemical reaction of 1,3-dithiane and Fe(0) can proceed with formal oxidative addition of a C−S bond. (Scheme 19).227,234,235 2.3.3. Strained Thioethers: Thiiranes and Thietes. Strained cyclic thioethers can be effective precursors to diiron dithiolato carbonyls, although the route is rarely used. It has

Table 4. Synthetic Results for Reactions of Thiols, Disulfides, and Thioethers with Fe(0) Carbonyls to Give Fe2(μ-SR)2(CO)6; (Reactions Were Conducted with 0.02 mol of Fe(0) Reagent in 100 mL of Decane at 80 °C)18 Fe3(CO)12

a

Fe2(CO)9

Fe(CO)5

organosulfur compound

equiv of S cmpd/equiv of Fe3(CO)12

time (h)

% yielda

equiv of S cmpd/equiv of Fe2(CO)9

time (h)

% yield

equiv of S cmpd/equiv of Fe(CO)5

time (h)

% yield

RSHb RSSRc RSRd

3:1 3:2 3:1

1 5 18

92 74 62

2:1 1:1 2:1

11 19 24

91 66 52

1:1 1:2 1:1

22 34 36

60 56 24

Yields are averages for Fe2(μ-SR)2(CO)6 isolated after chromatographic purification. bR = alkyl, benzyl, phenyl. cR = alkyl. dR = alkyl, phenyl. 7054


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The reactions of thiobenzophenones toward Fe(0) can often be rationalized by invoking the degradation of the thione to the thiosulfine Ar2CSS. The thiosulfine is a dipole that adds Ph2CS to give tetraphenyl-1,2,4-trithiolane.253,254 This trithiolane exists in equilibrium, rapid near 50 °C, with Ph2CS and, possibly, the thione-sulfide/dithiirane isomers of Ph 2CS2 (Scheme 21).245

Scheme 19. Activation of CH2(SR)2 Substrates by Fe(0)227,234,235

Scheme 21. Equilibria Relating Thiobenzophenone, Its Thiosulfine and Dithiirane Derivatives, and the Corresponding 1,2,4-Trithiolane253,254 long been known that iron carbonyls desulfurize thiiranes (episulfides).236−238 In fact, a good synthesis of Fe3(μS)2(CO)9 involves the reaction of Fe3(CO)12 and ethylene sulfide.239 Propylene sulfide reacts with Fe(CO)5 in the presence of the decarbonylating agent Me3NO to give good yields of the simplest chiral dithiolato complex (eq 14).240 Fe2(μ-edt)(CO)6 can be obtained less efficiently by reaction of thiirane-S-oxide and Fe3(CO)12 in hot THF.241 The fate of the oxo group in this obscure reaction was not determined. The deoxygenation of sulfur ligands is observed in other reactions of Fe3(CO)12.201 Reactions of thietanes, e.g., (CH2)3S, and iron carbonyls have not been reported. Thietes, unsaturated four-membered C3S rings, are valence isomers related to the highly reactive species thioacrolein (CH2CHCHS). These strained heterocycles ring-open upon treatment with iron carbonyls. The resulting thioacrolein complex (η4-CH2CHCHS)Fe(CO)3 represents an analogue of (η4-butadiene)Fe(CO)3. It undergoes decarbonylation to give the corresponding 36e− tetracarbonyl diiron dithiolate (Scheme 20).242

Thus, the formation of Fe2(μ-S2CPh2)(CO)6 from Ph2CS and Fe(0) carbonyls can be rationalized by invoking the oxidative addition of the dithiirane Ph2CS2.243,245,255,256 C−H activation occurs in these, as illustrated by one of the products in Scheme 22.243 Bulky aliphatic thiones (nonbulky dialkyl thiones are unstable with respect to oligomerization257) generally react with Fe(0) reagents to give diiron 1,1-dithiolates. These reactions have been proposed to proceed via 1,1-dithiolatodiiron complexes with two bridging CO ligands.255 Diiron dithiolates with a single bridging CO have rarely been observed,142 much less two bridging CO groups. Thus, these assignments should be viewed with caution. Potentially relevant to the pathway leading to Fe2(μS2CR2)(CO)6 from thioketones are studies on the reactions of a bulky thioketene with Fe2(CO)9. This reaction affords the Fe2(CO)6 adduct.258 This species reacts further with thioketene to give the 1,1-dithiolate with perfect stoichiometry (Scheme 23).213 2.4.2. Derivatives of CS2. In its reactions with metal carbonyls, carbon disulfide is often a source of esoteric products, which can inspire rational syntheses.259,260 A tetrairon derivative of methanetetrathiolate arises via the reaction of Fe3(CO)12 with a mixture of CS2 and PEt3 (Scheme 24).261 No yield was published, but it is probably very low. The structure was determined by X-ray crystallography. The related reaction of CS2 with Fe3(CO)12 gives a complex derived from C2S4, again in very low yield. A number of complexes are known with ligands derived from tetrathiooxalate,262,263 but this FeC2S4 species remains unique. As established by X-ray crystallography, it is a rare complex of an ethylenetetrathiolate (C2S44−) vs a tetrathiooxalate (C2S42−), featuring the unprecedented 1,1dithiolene bonding mode.264 2.4.3. Dithioesters and Related Di- and Trithiocarbonates. Upon treatment with PhCS2Me and related dithioester substrates, iron carbonyls give diiron adducts, not thiolates.265 As summarized in Linford and Raubenheimer’s review,221 the reactions of Fe(0) reagents with many acyclic trithiocarbonates (i.e., (RS)2CS) and xanthates (i.e., (RS)(RO)CS) proceed similarly.221 Ethylenetrithiocarbonate (SCS2C2H4) reacts with Fe2(CO)9 to give adducts of Fe2(CO)6 similar to those obtained from dithioesters. This initial species undergoes a complicated but efficient transformation to the diiron 1,1-dithiolate.266,267 These results are reminiscent of Behrens’s report on thioketenes serving as Sdonors and alkylidene precursor (Scheme 23). This parallel supported the notion that at least some diiron 1,1-dithiolato

Scheme 20. C−S Bond Scission by Fe(0) To Give Thioacrolein Complexes242

2.4. Reactions of Iron(0) Reagents and CS-Containing Reagents

Iron carbonyls are highly reactive toward the CS functionality221 (thioketones,243−245 dithioesters,209,246,247 and di- and trithiocarbonates248−250), giving a variety of products. Although this approach often suffers from low yields, it produces diiron dithiolates of distinctive structures. 2.4.1. Thiones. The case of diphenylcyclopropenethione is an instructive starting point for the discussion. Upon treatment with Fe2(CO)9, it forms a simple 1:1 adduct Fe(CO)4(SC3Ph2) in high yield. The efficiency of the reaction reflects the almost unique ability of this thione to serve as a unidentate Lewis base, a consequence of the aromatic character of the cyclopropenium cation.251 Otherwise thioketones typically attach to metals as η2-ligands, as in Pt(η2-SCPh2)(PPh3)2.252 7055


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Scheme 22. General Pattern for Reactions of Diarylthiones and the Tetraaryl 1,2,4-Trithiolanes with Iron Carbonyls243,255

Dithiooxamides (DTAs) are a family of potentially chelating organic thiocarbonyls.272 At room temperature in THF solution, they react with Fe2(CO)9 to initially give complexes of the type Fe(CO)3(κ2-DTA), wherein the DTA is proposed to function as a 4e− donor, S,S-chelated to Fe. Although not examined crystallographically, these deeply colored Fe(CO)3(κ2-DTA) complexes were well characterized spectroscopically and, in one case, obtained in analytical purity. These monomeric intermediates react further with Fe2(CO)9 to form, depending on the substituents, three types of diiron hexacarbonyl complexes, often in high yield (Scheme 25).271

Scheme 23. Efficient Pathway Connecting a Thioketene and a Diiron 1,1-Dithiolate258,213

Scheme 25. Products Generated by Reaction of Fe2(CO)9 with Dithiooxamide271 Scheme 24. Unusual Products Obtained from the Reactions of Fe3(CO)12 with CS2264,261

complexes arise by S-abstraction from R2CS functionality, not from dithiiranes. Dithiocarbonates (OC(SR)2) react with Fe2(CO)9 to give modest yields of the dithiolate complexes including those of edt, pdt, and ethylenedithiolates. An idealized equation for this transformation is given in eq 15. 2Fe2(CO)9 + (RS)2 CO → Fe2(SR)2 (CO)6 + 2Fe(CO)5 + 3CO

2.4.5. Miscellaneous Thiocarbonyl Derivatives. Thiocarbonyl complexes are known for Fe(0), e.g., Fe(CO)4CS, although derivatives of the type Fe2(μ-SR)2(CO)6−x(CS)x have not been reported. Reaction of Fe(CO)4CS with P(NMe2)3 results in complicated mixtures reflecting the multiple roles served by this very basic phosphine, which is an S-abstractor ligand and forms phosphoranes. One of the products, confirmed by X-ray crystallography, is Fe2[μ-S2C2(O)(P(NMe2)3)](CO)6,273 which is related to the diiron dithiooxamide species shown above.271 N-Sulfinylphenylhydrazine (PhNHNSO) is degraded by Fe2(CO)9 to give traces of Fe2(μ-SPh)2(CO)6.274

(15)

These reactions proceed via oxidative addition leading to the intermediate Fe2(μ-SR)(μ-C(O)SR)(CO)6.268 The latter was proposed to feature a μ-RSC(O) group. This intermediate should probably be reformulated with the acyl bridge μRSC(O) related to acyl thiolates Fe2(μ-SR)(μ-C(O)R)(CO)6 derived from acylation of [Fe2(μ-SR)(CO)7]− (section 3.7).169 From unsymmetrical dithiocarbonates (OC(SR)(SR′)), one can obtain the mixed thiolate complexes, sometimes selectively.268,269 A more efficient route to mixed dithiolates starts, however, with Fe2(μ-S2)(CO)6 (section 3.2). 2.4.4. Thioamides. Thioamides generally have not proven to be useful precursors to diiron(I) dithiolato carbonyls. Mercaptopyrimidine, which can be viewed as a tautomer of a thioamide, undergoes desulfurization by Fe(0).270 Tetramethylthiourea gives an adduct Fe(CO)4(SC(NMe2)2),255 analogous to Weiss’s cyclopropenethione Fe(CO)4(SCC2Ph2) described earlier.

2.5. Reactions of Iron(0) Reagents and Thioesters and Thioamides

Thioesters, compounds with the functionality RC(O)SR′, are intriguing substrates for reactions with iron carbonyls. Iron sulfide minerals are proposed to catalyze the formation of thioesters by a carbonylation pathway according to Wächter7056


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shäuser’s Iron−Sulfur Theory for the origin of life.275,276 Reactions of iron carbonyls with thioesters, however, have been examined only lightly. When Fe2(CO)9 is treated with chelating phosphine thioesters, one obtains diiron(II) dithiolates.277 The process appears to proceed via the chelate-assisted oxidative addition of the thioester to a single iron(0) center (Scheme 26).278 A pair

gives a few percent of dithiolate diiron hexacarbonyl complex.283 The related reaction of Fe3(CO)12, S8, and 1hexene (60 °C, 2 h) gives Fe4(μ-S)(μ-S2X)(CO)11 ((HS)2X = 1,2-hexylenedithiol) in 2% yield.284−286 Mixtures of Fe3(CO)12 and S8 even activate THF to produce traces of diiron complex derived from cis-tetrahydrofuran-2,3-dithiol.287,288 Further illustrative of this theme (and the diligence of these workers) is the reaction of Fe3(CO)12, S8, and cyclohexene, which again gives a variety of products in low yields.289 Thus, starting from 50 g of Fe3(CO)12, one obtains a variety of cyclohexyl/ cyclohexenyl−thiolato complexes.290−292 Fe3(CO)12 reacts with excess styrene and S8 at 60 °C to give the perthiolate complex Fe2(μ-S3CH2)(CO)6 (Scheme 27), in addition to the usual mixture of minor products (Fe2(μ-S2)(CO)6, Fe3(μ-S)2(CO)9, Fe2(μ-S2CO)(CO)6, and Fe2(μ-S2C2H3Ph)(CO)6).293−295 The related perthiolate Fe2(μ-S3CPh2)(CO)6 can be prepared by photoaddition of Fe2(μ-S2)(CO)6 (section 3.6). The formation of Fe2(μ-S3CH2)(CO)6 implies the scission of the CC bond in styrene. This unusual product is obtained only in low yields, unfortunately. The complex Fe2(μ-S3CH2)(CO)6 is a rare example of a perthiolate derivative of an diiron dithiolato carbonyl; other examples of μ-perthiolato diiron complexes include [Fe2(μ-S2-t-Bu)(μ-S)(CO)6]− and possibly [Fe2(μS2CF3)(μ-SCF3)(CO)6].194,296 Reaction of Fe3(CO)12 with S8 and benzalaniline (PhCHNPh) gives low yields of the 1,1dithiolate Fe2(μ-S2CHPh)(CO)6.297 This complex is more efficiently prepared (22%) by the reaction of PhCH(SH)2 with Fe3(CO)12.82 Photochemical reactions of Fe(CO)5, FcCCH, and S8 at low temperatures afford dithiolene complexes Fe2(μ-S2C2HFc)(CO)6 and the thioester Fe2(μ-S2C(O)C2HFc)(CO)6 (eq 16).298 The complex Fe2(μ-S2C2HFc)(CO)6 can also be obtained by microwave irradiation of toluene solution of Fe3(μ-S2)(CO)9 with FcCCH and Me3NO·2H2O.299

Scheme 26. Oxidative Addition of a Thioester by Fe(0)277

of the resulting acyliron thiolates condense concomitant with their decarbonylation, a reaction that can be reversed under high pressures of CO.277 The decarbonylation is consistent with the usual instability of ferrous tricarbonyl complexes. Pyridine-containing thioesters react with Fe(0) carbonyls to give thiolate/pyridyl-bridged diiron hexacarbonyl complexes.279 2.6. Reactions of Iron(0) Reagents, Elemental Sulfur, and Alkenes

One might expect that iron carbonyls, being zerovalent, would react readily with elemental sulfur to afford Fe−S−CO complexes and clusters. Such transformations, however, have not been demonstrated to be synthetically useful. This same reaction, however, affords the fundamental Fe−S−CO species Fe2(μ-S2CO)(CO)6, albeit in only 1% yield.280 This dithiocarbonate can be prepared in 45% yield by UV photolysis of a solution of Fe2(μ-S2)(CO)6 and CO.69 2.6.1. Alkene, Alkyne, and Imine Trapping Reactions. Although often proceeding in very low yields, the reaction of Fe(CO)5 (or Fe3(CO)12281) with sulfur in the presence of alkenes gives diiron dithiolate complexes, often unusual ones (Scheme 27).282 No reaction is expected between the alkene and the sulfur in the absence of the iron carbonyl. The reaction of trimethylvinylsilane with sulfur in the presence of Fe3(CO)12

2.6.2. Diene-Trapping Reactions. Diverse complexes are generated by the thermal reaction of dienes with iron carbonyls in the presence of S8, although again the yields are low. In the case of 1,3-cyclohexadiene, one obtains the expected 3cyclohexene-1,2-dithiolato complex. Also formed in this reaction is the trinuclear complex Fe3(η4-C6H8)(μ-S)(CO)8, a substituted derivative of Fe3(μ-S)(CO)10.300,301 Analogously, Fe3(CO)12, S8, and 1,5-cyclooctadiene gave the adduct Fe2(μS2C8H12)(CO)6 as well as the usual collection of iron carbonyls, Fe2(μ-S2)(CO)6, Fe3(μ-S)2(CO)9, and Fe3(μ-S)(CO)10 (Scheme 27). In addition to various Fe−S−CO clusters, the reaction of Fe3(CO)12, S8, and norbornadiene gives four isomers of Fe2(μ-S2C7H8)(CO)6 (Figure 12).302−307 Two of these species are chiral owing to the low symmetry of their dithiolate backbones.

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